1. Field of the Invention
This invention relates to diffusers for use with radial centrifugal compressors.
More particularly, this invention relates to two-row pipe diffusers.
In a further and more specific aspect, the instant invention concerns configurations for controlling the amount of boundary layer build-up and increasing the rate of diffusion in a pipe diffuser.
2. Description of the Prior Art
Gas turbine engines and various other types of turbomachinery commonly employ centrifugal compressors in which a rotating impeller is arranged to add kinetic energy by accelerating the gas flow. A diffuser is located downstream of the impeller for reducing the velocity of the outflowing gas and thus converting the kinetic energy into static pressure. The overall performance of the compressor, and therefore of the engine itself, is influenced greatly by the effectiveness with which the diffuser makes this kinetic-energy-to-pressure conversion. Accordingly, it is very important when designing a diffuser to select a configuration in which frictional and other types of losses which reduce the static pressure recovery of the diffuser are minimized.
The simplest type of diffuser is the vaneless diffuser, which consists merely of an open annular area surrounding the periphery of the compressor impeller. In this configuration, the gas flows in a natural spiral path from the tip of the impeller blade to the outer circumference of the diffuser. Because the natural flow path is rather long and circuitous, losses due to friction and the like in a vaneless diffuser tend to be great, and static pressure recovery is poor.
A second type of diffuser, which remedies many of the problems in the veneless diffuser, is the vaned or bladed diffuser. In vaned diffusers, a plurality of passages are formed between rows or islands of vanes. The passages are generally rectangular in cross-section. These passages guide the fluid from the tip of the impeller blade to the outer circumference of the diffuser by a shorter path than is permitted by the vaneless arrangement, and thus reduce the losses due to friction.
Nonetheless, vaned or bladed diffusers suffer from a number of shortcomings. One problem is that boundary layers tend to build up on the walls along the corners of the rectangular flow passages. This tends to promote detachment of the boundary layer and decreases the effectiveness of static pressure recovery. Another problem is that vaned diffusers are difficult to manufacture. Great care must be taken to provide each of the vanes with a profiled leading edge for preventing or reducing shock waves at the inlet of the diffuser. The leading edge must be very precisely machined, and the vanes carefully assembled relative to one another and to the impeller in order to avoid losses and to obtain geometric balance. As a result, production costs are high.
In response to the aforementioned problems, a third type of diffuser, known as the pipe, or channel, diffuser was developed. The pipe diffuser comprises a solid annular member surrounding the impeller and having a plurality of passageways extending therethrough.
Perhaps the best-known example of a pipe diffuser is that shown in U.S. Pat. No. 3,333,762 to Vrana. Each of the passageways of Vrana has a curvilinear cross-section and extends through the annular member in such a manner as to be tangent to a common circle substantially equal in diameter to the periphery of the impeller. Each passageway is disposed so that it intersects with the next adjacent passageway near the inner circumference of the annular member. The spacing and geometry of the passageways are such that a plurality of overlapping elliptical openings surround the periphery of the impeller blades, thus forming an undulating, or "scalloped" leading edge which prevents the occurrence of shock waves.
The pipe diffuser of Vrana and other diffusers of similar construction are generally more effective at producing high pressure rises than are the conventional vaned and vaneless diffusers discussed above. The performance of this type of pipe diffuser is good even when the Mach number of the gas discharged from the impeller is high. However, when the mass flow rate drops below design point, the passageways tend to become blocked due to boundary layer build-up, thus leading to diffuser stall and surge. These problems can become especially severe at low inlet pressure and low inlet temperature operating conditions in the compressor of a turbine engine.
It would be highly advantageous, therefore, to remedy the foregoing and other deficiencies inherent in the prior art.
Accordingly, it is an object of the present invention to provide means for controlling boundary layer growth in a pipe diffuser.
Another object of the invention is the provision of a pipe diffuser with an improved rate of diffusion.
And another object of the invention is to provide a pipe diffuser capable of optimum static pressure recovery at design point.
Still another object of the invention is the provision of a pipe diffuser capable of high performance over a wide flow range.
Yet another object of the invention is to provide a pipe diffuser with improved stall characteristics.
Yet still another object of the invention is the provision of a diffuser configuration which minimizes losses due to friction and the like.
And a further object of the invention is to provide a diffuser with optimum leading edge geometry, for reducing entry shock during subsonic, transonic, and supersonic flow.
And still a further object of the invention is the provision of a diffuser, according to the foregoing, which can be simply, economically, and accurately manufactured.